High-strength zirconium-niobium alloy forgings and methods of making the same

Abstract

The application belongs to the technical field of nuclear-grade zirconium alloy materials, and particularly relates to a high-strength zirconium-niobium alloy forge piece and a manufacturing method thereof. The high-strength zirconium-niobium alloy forge piece is composed of the following components in percentage by mass: Nb 2.45% to 2.85%, O 0.09% to 0.13%, and the balance of unavoidable impurities and Zr, wherein the impurities include Hf with a content less than or equal to 50 ppm. The high-strength zirconium-niobium alloy forge piece has high room-temperature and high-temperature tensile properties, and the high corrosion resistance is retained through performance heat treatment in the preparation process. Therefore, when the forge piece is applied to the construction of the pressure boundary in the nuclear energy comprehensive application field, the container thickness can be reduced, and high neutron economy is obtained.

Description

Technical Field

[0001] This invention belongs to the field of nuclear-grade zirconium alloy materials technology, specifically relating to a high-strength zirconium-niobium alloy forging and its manufacturing method. Background Technology

[0002] The information disclosed in this background section is intended only to enhance understanding of the overall background of the invention and is not necessarily to be construed as an admission or in any way implying that such information constitutes prior art known to those skilled in the art.

[0003] In the field of nuclear power and integrated nuclear energy applications, nuclear-grade zirconium alloy materials used in the construction of pressure-bearing boundaries (such as pressure pipes of heavy water reactors and various fuel claddings) under high temperature and high pressure conditions must have good neutron economy. The purpose is to ensure that when neutrons pass through the zirconium alloy material, the material absorbs fewer neutrons and more neutrons pass through the zirconium alloy to maintain the continuous occurrence of the chain reaction or to carry out other nuclear applications.

[0004] There are two main types of existing methods for improving the neutron economy of zirconium alloys:

[0005] a) Optimize alloy composition. Zirconium and its main alloying elements (such as niobium, tin, and oxygen) have small neutron absorption cross sections, while some impurity elements (such as B, Cd, C, Hf, W, and Pb) have large neutron absorption cross sections. Therefore, the content of these impurity elements in zirconium alloys must be controlled at extremely low levels.

[0006] b) Reduce alloy thickness. For nuclear-grade zirconium alloys of the same composition, a thinner pressure-bearing boundary wall allows more neutrons to pass through, improving neutron economy. Increasing the strength of the alloy material is an effective way to reduce alloy thickness. Taking a pressure vessel with a circular cross-section and subjected to internal pressure as an example, without considering corrosion and wear allowances, the pressure vessel wall thickness δ has the following relationship with the allowable stress S, vessel diameter D, and vessel design pressure P:

[0007]

[0008] With a constant safety factor, the higher the strength of the material, the higher the allowable stress. Therefore, the higher the strength of zirconium alloy, the thinner its wall thickness during use, and the more neutrons (neutron flux) pass through the alloy per unit time. The high strength of zirconium alloy also has another advantage. In equation (1), the container diameter refers to the mean diameter of the container wall thickness. Therefore, thinning the wall thickness means that the container volume increases, providing more space for the contents to react at the pressure boundary.

[0009] Forgings are one of the main material types used in the construction of pressure vessels, and can be used as flanges for rolled-welded pressure vessels and shells and heads for forged-welded pressure vessels. Currently, the standard commonly used in industry for manufacturing zirconium alloy forgings is GB / T 30568, which is equivalent to ASTM B493 in the United States. This specification defines three non-nuclear grade forgings: R60702 (pure zirconium), R60704 (Zr-Sn alloy), and R60705 (Zr-Nb alloy). Currently, the specified ductile elongation strength requirements for zirconium alloy forgings at room temperature are generally 200–400 MPa, while the tensile strength requirements are 350–490 MPa.

[0010] According to equation (1), selecting nuclear-grade zirconium alloys with higher strength is key to improving the neutron economy of zirconium alloy pressure-bearing boundaries. Currently, nuclear-grade zirconium alloy forgings are typically in bar form, conforming to ASTM B351 and its equivalent standards. These standards specify a plastic elongation strength requirement of 200–400 MPa and a tensile strength requirement of 350–490 MPa at room temperature, limiting further improvements in the neutron economy of nuclear-grade zirconium alloy pressure-bearing boundaries. In certain high-temperature and high-pressure applications where high neutron economy is required for pressure-bearing boundaries, existing nuclear-grade zirconium alloy forgings cannot meet the requirements. Summary of the Invention

[0011] To address the shortcomings of existing technologies, the present invention aims to provide a high-strength zirconium-niobium alloy forging and its manufacturing method. The high-strength zirconium-niobium alloy forging of the present invention possesses high room temperature and high temperature tensile properties, and retains high corrosion resistance through performance heat treatment during the manufacturing process.

[0012] To achieve the above objectives, the present invention is implemented through the following technical solution:

[0013] In a first aspect, the present invention provides a high-strength zirconium-niobium alloy forging composed of the following components by mass percentage: Nb 2.45% to 2.85%, O 0.09% to 0.13%, with the balance being unavoidable impurities and Zr, said impurities including Hf with a content of less than or equal to 50 ppm.

[0014] Preferably, the composition consists of the following components in mass percentage: Nb 2.50%–2.80%, O 0.10%–0.13%, with the balance being unavoidable impurities and Zr.

[0015] Preferably, the impurities include Hf at a content of less than or equal to 50 ppm, Fe at a content of less than or equal to 0.13%, and C at a content of less than or equal to 0.008%.

[0016] More preferably, the impurities include Hf at a content of less than or equal to 50 ppm, Fe at 0.065% to 0.13% and C at 0.004% to 0.008%.

[0017] Preferably, at room temperature, the specified plastic elongation strength R of the high-strength zirconium-niobium alloy forging is... p0.2 ≥485MPa, tensile strength R m ≥710MPa, elongation ≥16%; at 350℃, the specified plastic elongation strength R of the high-strength zirconium-niobium alloy forging p0.2 ≥312MPa, tensile strength R m ≥453MPa, elongation ≥16%.

[0018] Preferably, at 350°C, the corrosion resistance of the high-strength zirconium-niobium alloy forging meets the following requirement: after immersing the high-strength zirconium-niobium alloy forging in Class A water for 72 hours in a steam environment at 400°C and a pressure of 10.3 MPa, the weight gain does not exceed 35 mg / dm³. 2 Its surface is grayish-black and shiny.

[0019] Secondly, a method for preparing a high-strength zirconium-niobium alloy forging as described in the first aspect includes the following steps:

[0020] S1. Nuclear-grade zirconium raw materials, Zr-Nb master alloy and zirconium oxide are vacuum arc remelted to obtain an ingot whose composition meets the requirements of high-strength zirconium-niobium alloy forgings. The ingot is then forged to obtain a billet.

[0021] S2. The billet is subjected to β-phase solid solution and quenched, then shaped and the flash is removed by machining to obtain a zirconium-niobium alloy rough forging.

[0022] S3. Quench the rough forging of zirconium-niobium alloy in the α+β phase region, anneal it after quenching, and then machine it to obtain the high-strength zirconium-niobium alloy forging with the final dimensions.

[0023] Preferably, the nuclear-grade zirconium raw material has an Hf content of <50ppm, including nuclear-grade sponge zirconium, nuclear-grade zirconium alloy recycled material, or nuclear-grade master alloy; the Zr-Nb master alloy has an Nb content of higher than 2.5%.

[0024] Preferably, during the initial forging, the material is heated to a temperature not lower than 810°C, and the final forging temperature not higher than 650°C. The initial forging can be performed in multiple passes. During the initial forging, the material can be heated to the β phase region or to the (α+β) phase region, with the final forging temperature not higher than 650°C. When heating to the β phase region, the temperature is not lower than 910°C. When heating to the (α+β) phase region, the temperature is not lower than 810°C.

[0025] Preferably, during β-phase solid solution treatment, the heating temperature is >910℃ and the holding time is 1.5 to 3 hours.

[0026] Preferably, the α+β phase region quenching is specifically performed by heating to 750-910℃, holding at that temperature, and then immersing in water for cooling, with a holding time of 1-2 hours; the annealing is specifically performed by heating to 450-550℃, holding at that temperature for no less than 6 hours, and then furnace cooling to room temperature.

[0027] Thirdly, as described in the first aspect, the application of high-strength zirconium-niobium alloy forgings in the construction of pressure-bearing boundaries in the field of integrated nuclear energy applications.

[0028] The beneficial effects achieved by one or more technical solutions of the present invention are as follows:

[0029] The high-strength zirconium-niobium alloy forgings of this invention ensure that the Hf element content in the alloy is less than 50 ppm while possessing high room temperature and high temperature tensile properties. Therefore, when this forging is used in the construction of pressure-bearing boundaries in the field of integrated nuclear energy applications, the container thickness can be reduced, resulting in higher neutron economy.

[0030] This invention significantly improves the strength of forgings while maintaining high corrosion resistance by subjecting them to special performance heat treatment.

[0031] The high-strength zirconium-niobium alloy forgings of this invention have an allowable pressure of 202 MPa at room temperature when used for the construction of nuclear Class 2 and 3 pressure-bearing boundaries, and an allowable pressure of 236 MPa at room temperature when used for the construction of nuclear Class 1 pressure-bearing boundaries. Compared with ASTM B351 R60901 alloy, the allowable pressure is significantly increased, the wall thickness of the pressure-bearing boundary is significantly reduced, and the neutron flux rate of the sample within the pressure-bearing boundary is increased. Attached Figure Description

[0032] The accompanying drawings, which form part of this invention, are used to provide a further understanding of the invention. The illustrative embodiments of the invention and their descriptions are used to explain the invention and do not constitute an improper limitation of the invention.

[0033] Figure 1 This is a schematic diagram of the pressure boundary structure of a nuclear-grade container. Detailed Implementation

[0034] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments and comparative examples.

[0035] The chemical composition of the high-strength zirconium-niobium alloy forgings of the present invention is shown in Table 1.

[0036] Table 1 Chemical composition of high-strength zirconium-niobium alloy forgings

[0037]

[0038]

[0039] Example 1

[0040] The high-strength zirconium-niobium alloy forging of this embodiment is composed of the following components by mass percentage: Nb 2.50%, O 0.10%, with the balance being unavoidable impurities and Zr, the impurity contents of which are shown in Table 1.

[0041] The method for preparing the high-strength zirconium-niobium alloy forgings in this embodiment includes the following steps:

[0042] Nuclear-grade sponge zirconium with Hf content <50ppm and Zr-Nb master alloy are vacuum arc remelted to obtain an ingot whose composition meets the requirements of high-strength zirconium-niobium alloy forgings. The ingot is then forged to obtain a billet.

[0043] The billet is subjected to β-phase solid solution and quenching, and then formed and machined to obtain zirconium-niobium alloy rough forgings;

[0044] The zirconium-niobium alloy rough forging is heated to 880℃ and held at that temperature before being cooled in water. It is then heated to 500℃ and held at that temperature before being furnace cooled to room temperature. After finishing, a high-strength zirconium-niobium alloy forging is obtained.

[0045] Example 2

[0046] The high-strength zirconium-niobium alloy forging of this embodiment is composed of the following components by mass percentage: Nb 2.60%, O 0.10%, with the balance being unavoidable impurities and Zr, the impurity contents of which are shown in Table 1.

[0047] The method for preparing the high-strength zirconium-niobium alloy forgings in this embodiment includes the following steps:

[0048] Nuclear-grade sponge zirconium with Hf content <50ppm and Zr-Nb master alloy are vacuum arc remelted to obtain an ingot whose composition meets the requirements of high-strength zirconium-niobium alloy forgings. The ingot is then forged to obtain a billet.

[0049] The billet is subjected to β-phase solid solution and quenching, and then formed and rough machined to obtain zirconium-niobium alloy rough forgings;

[0050] The zirconium-niobium alloy rough forging is heated to 850℃ and held at that temperature before being cooled in water. It is then heated to 510℃ and held at that temperature before being furnace cooled to room temperature. After finishing, a high-strength zirconium-niobium alloy forging is obtained.

[0051] Example 3

[0052] The high-strength zirconium-niobium alloy forging of this embodiment is composed of the following components by mass percentage: Nb 2.70%, O 0.10%, with the balance being unavoidable impurities and Zr, the impurity contents of which are shown in Table 1.

[0053] The method for preparing the high-strength zirconium-niobium alloy forgings in this embodiment includes the following steps:

[0054] Nuclear-grade sponge zirconium with Hf content <50ppm and Zr-Nb master alloy are vacuum arc remelted to obtain an ingot whose composition meets the requirements of high-strength zirconium-niobium alloy forgings. The ingot is then forged to obtain a billet.

[0055] The billet is subjected to β-phase solid solution and quenching, and then formed and rough machined to obtain zirconium-niobium alloy rough forgings;

[0056] The zirconium-niobium alloy rough forging is heated to 830℃ and held at that temperature before being cooled in water. It is then heated to 550℃ and held at that temperature before being furnace cooled to room temperature. After finishing, a high-strength zirconium-niobium alloy forging is obtained.

[0057] The tensile properties of the high-strength zirconium-niobium alloy forgings obtained in the above embodiments are shown in Table 2.

[0058] Table 2 Tensile properties of high-strength zirconium-niobium alloy forgings

[0059]

[0060]

[0061] The weight gain corrosion test for the high-strength zirconium-niobium alloy forgings of this invention is as follows: The corrosion test specimens should be taken from near the tensile specimens. The corrosion test is conducted in a steam environment at 400°C (pressure 10.3 MPa) and should comply with ASTM G2 specifications. Grade A water is used during the test, and the maximum oxygen content during the test is 0.03 cm³. 3 / kg, oxygen content should be controlled by venting the autoclave before reaching the target test temperature. After 72 hours, the weight gain of the sample should not exceed 35 mg / dm³. 2 The corroded sample surface is grayish-black and shiny. The high-strength zirconium-niobium alloy forgings of this invention can all meet the requirements of the weight-gain corrosion test and exhibit high corrosion resistance.

[0062] Example 4

[0063] A certain nuclear secondary pressure vessel is mainly used for experimental research on the effects of neutrons on the samples contained inside the vessel. A schematic diagram of the device is attached. Figure 1 As shown. Neutrons need to penetrate the wall thickness of the pressure vessel and further interact with the sample. To simulate the sample's actual operating environment, it needs to be immersed in a high-temperature, high-pressure medium (such as water). The design temperature of the pressure vessel is 350℃, the design pressure is 17.2MPa, and the water pressure is 21.6MPa. Due to limitations in the experimental setup, the mid-diameter of its pressure-bearing boundary must not exceed 100mm.

[0064] The original plan was to use ASTM B351 R60901 alloy as the pressure boundary. According to the principles specified in the ASME BPVC standard, its allowable stress at room temperature is 128 MPa. Using the hydrostatic test condition as the boundary condition for design verification, the wall thickness of the pressure boundary was found to be at least 9.22 mm.

[0065] When constructing, high-strength zirconium-niobium alloy forgings from Examples 1-3 of this invention are selected. The allowable stress at room temperature is 202 MPa, resulting in a bearing boundary wall thickness of 5.65 mm. Further, at 350 °C, the design is checked according to formula (1), and the bearing boundary wall thickness is found to be at least 7.14 mm.

[0066] Calculations show that reducing the wall thickness from 9.22 mm to 7.14 mm can improve the neutron flux rate experienced by the sample within the pressure-bearing boundary. Simultaneously, the inner diameter of the pressure vessel is increased by 2.08 mm, thus enabling the vessel to meet the irradiation test requirements of a certain type of nuclear fuel pilot assembly.

[0067] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the present invention by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for preparing a high-strength zirconium-niobium alloy forging, characterized in that, Includes the following steps: S1. Nuclear-grade zirconium raw materials, Zr-Nb master alloy and zirconium oxide are vacuum arc remelted to obtain an ingot whose composition meets the requirements of high-strength zirconium-niobium alloy forgings. The ingot is then forged to obtain a billet. S2. The billet is subjected to β-phase solid solution and quenched, then shaped and the flash is removed by machining to obtain a zirconium-niobium alloy rough forging. S3. Quench the rough forging of zirconium-niobium alloy in the α+β phase region, anneal it after quenching, and then machine it to obtain the high-strength zirconium-niobium alloy forging with the final dimensions. When performing β-phase solid solution treatment, the heating temperature is >910℃ and the holding time is 1.5~3 hours; The α+β phase region quenching process involves heating to 750~910 ℃, holding at that temperature, and then immersing in water for cooling for 1~2 hours; the annealing process involves heating to 450~550 ℃, holding at that temperature for at least 6 hours, and then furnace cooling to room temperature. The high-strength zirconium-niobium alloy forgings are composed of the following components by mass percentage. Composition: Nb 2.45%–2.85%, O 0.09%–0.13%, balance being unavoidable impurities and Zr, including Hf at a content of less than or equal to 50 ppm.

2. The preparation method according to claim 1, characterized in that, The nuclear-grade zirconium raw material has an Hf content of <50ppm, including nuclear-grade sponge zirconium, nuclear-grade zirconium alloy recycled material, or nuclear-grade master alloy; the Zr-Nb master alloy has an Nb content of higher than 2.5%.

3. The preparation method according to claim 1, characterized in that, During the initial forging, the material is heated to a temperature not lower than 810℃, and the final forging temperature is not higher than 650℃.

4. The preparation method according to claim 1, characterized in that, It consists of the following components by mass percentage: Nb 2.50%–2.80%, O 0.10%–0.13%, with the balance being unavoidable impurities and Zr.

5. The preparation method according to claim 1, characterized in that, The impurities include Hf at a content of less than or equal to 50 ppm, Fe at a content of less than or equal to 0.13%, and C at a content of less than or equal to 0.008%.

6. The preparation method according to claim 5, characterized in that, The impurities include Hf at a content of less than or equal to 50 ppm, Fe at a content of 0.065% to 0.13% and C at a content of 0.004% to 0.008%.

7. The preparation method according to claim 1, characterized in that, At room temperature, the specified plastic elongation strength R of the high-strength zirconium-niobium alloy forging p0.2 ≥485 MPa, tensile strength R m ≥710 MPa, elongation ≥16%; at 350 °C, the specified plastic elongation strength R of the high-strength zirconium-niobium alloy forging. p0.2 ≥312 MPa, tensile strength R m ≥453 MPa, elongation ≥16%.

8. The preparation method according to claim 1, characterized in that, At 350 °C, the corrosion resistance of the high-strength zirconium-niobium alloy forging meets the following requirement: after immersing the high-strength zirconium-niobium alloy forging in Class A water for 72 hours in a steam environment at 400 °C and a pressure of 10.3 MPa, the weight gain does not exceed 35 mg / dm³. 2 Its surface is grayish-black and shiny.

9. The application of high-strength zirconium-niobium alloy forgings obtained by the preparation method according to any one of claims 4-8 in the pressure boundary of the construction of integrated nuclear energy applications.

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